So today, with the drive accessed through Samba, I ran on my computer a scan on the DietPi/HC2 drive and an external drive (using FreeFIleSync) to compare my DietPi/HC2 media folder with a backup of media I keep on the external. Not long after I noticed that my Plex clients couldn't load any media, so I thought something crashed with Plex Media Server. I SSH'd into the DietPi/HC2 and sent the reboot command.
And that's the last I ever heard from her...

Now, the LEDs appear normal with activity on the ethernet light and the blue 'heartbeat' LED giving its double blink. I can't SSH into the device (and no hdmi on this board), and looking in my router admin I see that it is not seen as connected to the network. I tried flashing DietPi to a different microSD, and that booted up fine, is visible to the router, and I can SSH to it, so it doesn't seem to be a network or hardware issue. What could be 'corrupted' on that microSD that causes it to no longer show up in the network?
So is there anything I can try to get back the installation I had yesterday?